This invention relates to a unidirectional self-starting synchronously operable motor. British Patent Specification No. 876576 discloses such a motor having a stator with two parts each stator part having a plurality of pole teeth which interdigitate with the pole teeth of the other stator part, some of the poles constituting unshaded or main poles and the other poles being auxiliary poles coupled by shading means such that the axialiary pole teeth will produce a flux phase-shifted by .phi. electrical degrees from the fluxes produced by the main pole teeth of the same polarity, and in the assembled stator the interdigitated teeth being in alternate groups of main pole teeth and auxiliary pole teeth whilst the groups of auxiliary pole teeth are angularly spaced from a symmetrical position to define the direction of starting and operation of the motor.
It is a characteristic of that motor that the number of pole teeth is less than the theoretical number n of poles, where n in that case equals number of rotor poles and is twice the number of cycles per second of the motor supply divided by the number of rotor revolutions per second. It is the mean pole pitch or pole set pitch within the groups which determines that speed, from which it follows that n can be defined by the pole pitch or pole set pitch.
Thus, n can be defined as the integer which is substantially equal to 360 divided by the mean pole pitch angle which exists within the majority of pole sets within the groups.
The reduced number of poles in this known motor gives rise to the situation in which the stator has fewer main pole teeth than auxiliary pole teeth to enable the groups of auxiliary pole teeth to adopt their position spaced from the symmetrical position without overlapping main pole teeth, i.e., a main pole teeth is omitted where such overlap would occur. This tends to equalize the flux from the main pole teeth with that from the auxiliary pole teeth, but nevertheless it is felt that the overall efficiency must be reduced by eliminating main poles, as these poles determine the available power of the motor, and by upsetting the natural alternation of the stator poles.
It is thus an object of the invention to improve such synchronous motors.
The object is achieved according to one aspect of the invention in that the number n of stator pole teeth is an integer substantially equal to 360 divided by the mean pole pitch angle which exists within the majority of the pole sets within the groups, that number n being accommodated by virtue of the pole teeth at the trailing end of the at least one group of shaded pole teeth and at the leading end of the at least one group of unshaded pole teeth being dimensioned and positioned so that each pole tooth is accommodated between, and without contact with, its two adjacent pole teeth, the number of said groups being x such that the number of pole teeth in any of said groups is an integer in the range from n/x - 1 to n/x +1, and said parts together provide a continuous alternating sequence of north and south poles. It will be seen that each group contains n/x teeth plus or minus one tooth and this is advantageous in that it provides an optimum or near optimum number of unshaded poles to maintain a reasonable torque.
If it is found that the auxiliary and main pole fluxes are undesirably unbalanced as to phase, it is proposed to add to the stator a magnetic shunt which has the effect of increasing .phi.. This shunt is additional to n pole teeth and, though it might be constructed in the manner of a pole tooth, it is not regarded herein as a pole tooth. This is achieved according to another aspect of the invention in that the number n of stator pole teeth is an integer substantially equal to 360 divided by the main pole pitch angle which exists within the majority of the pole sets within the groups, that number n being accommodated by virtue of the pole teeth at the trailing end of the at least one group of shaded pole teeth and at the leading end of the at least one group of unshaded pole teeth being dimensioned and positioned so that each pole tooth is accommodated between, and without contact with, its two adjacent pole teeth, the number of said group being x such that the number of pole teeth in any said groups is an integer in the range from n/x - 1 to n/x + 1, and said at least one group of shaded pole teeth has associated with it a shaded magnetic shunt substantially extending from one of said stator parts towards the other. As a consequence of the shunt extending completely to or near to the other stator part, the net driving field of that shunt is zero or negligible in comparison to that of said pole teeth.
In one embodiment, the magnetic shunt extends completely from one stator part to the other. Furthermore, with more than one auxiliary group, each such group of auxiliary pole teeth may have associated with it such a magnetic shunt. The or each shunt may be formed in the manner of a pole tooth but does not act as a pole tooth, and is not regarded as a pole tooth for the purposes of this specification, because the net driving effect of the shunt is zero or negligible in comparison with that of the proper teeth.
There is preferably a magnetic shunt at the leading end of each auxiliary group and advantageously it does not overlap the leading pole tooth of that auxiliary group or the next trailing pole tooth of the adjacent main group. The number of rotor poles is preferably equal to the number of stator poles, of course ignoring any shunt that may have been added.
It has also been found that the efficiency or power of a synchronous machine depends upon the field strength of its rotor. Such rotors have on occasions been made of isotropic ferrite but the field strength of such rotors is nevertheless limited because of their isotropic property.
An improved synchronous machine is provided according to another aspect of the invention with a permanent magnet rotor which is a radially polarized anisotropic ferrite rotor.
A preferred embodiment of synchronous motor incorporates all the afore-mentioned aspects.